1. Field of the Invention
This invention relates to an electron-beam generating device having a plurality of matrix-wired cold cathode elements and to a method of driving the device. The invention further relates to an image forming apparatus to which the electron-beam generating device is applied, particularly a display apparatus using phosphors as image forming members.
2. Description of the Related Art
Two types of elements, namely thermionic cathode elements and cold cathode elements, are known as electron emission elements. Examples of cold cathode elements are surface-conduction electron emission elements, electron emission elements of the field emission type (abbreviated to xe2x80x9cFExe2x80x9d below) and metal/insulator/metal type (abbreviated to xe2x80x9cMIMxe2x80x9d below).
An example of the surface-conduction electron emission element is described by M. I. Ellinson, Radio. Eng. Electron Phys., 10, 1290 (1965). There are other examples as well, as will be described later.
The surface-conduction electron emission element makes use of a phenomenon in which electron emission is produced in a small-area thin film, which has been formed on a substrate, by passing a current parallel to the film surface. Various examples of this surface-conduction electron emission element have been reported. One relies upon a thin film of SnO2 according to Ellinson, mentioned above. Other examples use a thin film of Au [G. Dittmer: xe2x80x9cThin Solid Filmsxe2x80x9d, 9, 317 (1972)]; a thin film of In2O3/SnO2 (M. Hartwell and C. C. G. Fonstad: xe2x80x9cIEEE Trans. E.D. Conf.xe2x80x9d, 519 (1975); or a thin film of carbon (Hisashi Araki, et al: xe2x80x9cShinkuuxe2x80x9d, Vol. 26, No. 1, p. 22 (1983).
FIG. 1 is a plan view of the element according to M. Hartwell, et al., described above. This element construction is typical of these surface-conduction electron emission elements. As shown in FIG. 1, numeral 3001 denotes a substrate. Numeral 3004 denotes an electrically conductive thin film comprising a metal oxide formed by sputtering. The conductive film 3004 is subjected to an electrification process referred to as xe2x80x9cenergization formingxe2x80x9d, described below, whereby an electron emission portion 3005 is formed. The spacing L in FIG. 1 is set to 0.5xcx9c1 mm, and the spacing W is set to 0.1 mm. For the sake of illustrative convenience, the electron emission portion 3005 is shown to have a rectangular shape at the center of the conductive film 3004. However, this is merely a schematic view and the actual position and shape of the electron emission portion are not represented faithfully here.
In above-mentioned conventional surface-conduction electron emission elements, especially the element according to Hartwell, et al., generally the electron emission portion 3005 is formed on the conductive thin film 3004 by the so-called xe2x80x9cenergization formingxe2x80x9d process before electron emission is performed. According to the forming process, a constant DC voltage or a DC voltage which rises at a very slow rate on the order of 1 V/min is impressed across the conductive thin film 3004 to pass a current through the film, thereby locally destroying, deforming or changing the property of the conductive thin film 3004 and forming the electron emission portion 3005, the electrical resistance of which is very high. A fissure is produced in part of the conductive thin film 3004 that has been locally destroyed, deformed or changed in property. Electrons are emitted from the vicinity of the fissure if a suitable voltage is applied to the conductive thin film 3004 after energization forming.
Known examples of the FE type are described in W. P. Dyke and W. W. Dolan, xe2x80x9cField emissionxe2x80x9d, Advances in Electron Physics, 8, 89 (1956), and in C. A. Spindt, xe2x80x9cPhysical Properties of Thin-Film Field Emission cathodes with Molybdenum Conesxe2x80x9d, J. Appl. Phys., 47, 5248 (1976).
A typical example of the construction of an FE-type element is shown in FIG. 2, which is a sectional view of the element according to Spindt, et al., described above. The element includes a substrate 3010, emitter wiring 3011 comprising an electrically conductive material, an emitter cone 3012, an insulating layer 3013 and a gate electrode 3014. The element is caused to produce a field emission from the tip of the emitter cone 3012 by applying an appropriate voltage across the emitter cone 3012 and gate electrode 3014.
In another example of the construction of an FE-type element, the stacked structure of the kind shown in FIG. 2 is not used. Rather, the emitter and gate electrode are arranged on the substrate in a state substantially parallel to the plane of the substrate.
A known example of the MIM type is described by C. A. Mead, xe2x80x9cOperation of Tunnel Emission Devicesxe2x80x9d, J. Appl. Phys., 32, 646 (1961). FIG. 3 is a sectional view illustrating a typical example of the construction of the MIM-type element. The element includes a substrate 3020, a lower electrode 3021 consisting of a metal, a thin insulating layer 3022 having a thickness on the order of 100 xc3x85, and an upper electrode 3023 consisting of a metal and having a thickness on the order of 80xcx9c300 xc3x85. The element is caused to produce a field emission from the surface of the upper electrode 3023 by applying an appropriate voltage across the upper electrode 3023 and lower electrode 3021.
Since the above-mentioned cold cathode element makes it possible to obtain an electron emission at a lower temperature in comparison with a thermionic cathode element, a heater for applying heat is unnecessary. Accordingly, the structure is simpler than that of the thermionic cathode element and it is possible to fabricate elements that are finer. Further, even though a large number of elements are arranged on a substrate at a high density, problems such as fusing of the substrate do not readily arise. In addition, the cold cathode element differs from the thermionic cathode element in that the latter has a slow response speed because it is operated by heat produced by a heater. Thus, an advantage of the cold cathode element is a quicker response speed.
For these reasons, extensive research into applications for cold cathode elements is being carried out.
By way of example, among the various cold cathode elements, the surface-conduction electron emission element is particularly simple in structure and easy to manufacture and therefore is advantageous in that a large number of elements can be formed over a large area. Accordingly, research has been directed to a method of arraying and driving a large number of elements, as disclosed in Japanese Patent Application. Laid-Open (Kokai) No. 64-31332, filed by the assignee of the present invention.
Applications of surface-conduction electron emission elements that have been researched are image forming apparatus such as image display apparatus and image recording apparatus, charged beam sources, etc.
As for applications to image display apparatus, research has been conducted with regard to such an apparatus using, in combination, surface-conduction type electron emission elements and phosphors which emit light in response to irradiation with an electron beam, as disclosed, for example, in the specifications of U.S. Pat. No. 5,066,883 and Japanese Patent Application Laid-Open (KOKAI) Nos. 2-257551 and 4-28137 filed by the assignee of the present invention. The image display apparatus using the combination of the surface-conduction type electron emission elements and phosphors is expected to have characteristics superior to those of the conventional image display apparatus of other types. For example, in comparison with. a liquid-crystal display apparatus that have become so popular in recent years, the above-mentioned image display apparatus emits its own light and therefore does not require back-lighting. It also has a wider viewing angle.
A method of driving a number of FE-type elements in a row is disclosed, for example, in the specification of U.S. Pat. No. 4,904,895 filed by the present applicant. A flat-type display apparatus reported by Meyer et al., for example, is known as an example of an application of an FE-type element to an image display apparatus. [R. Meyer: xe2x80x9cRecent Development on Microtips Display at LETIxe2x80x9d, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahara, pp. 6xcx9c9, (1991).]
An example in which a number of MIM-type elements are arrayed in a row and applied to an image display apparatus is disclosed in the specification of Japanese Patent Application Laid-Open No. 3-55738 filed by the assignee of the present invention.
Under these circumstances, the inventors have conducted exhaustive research with regard to multiple electron sources. FIG. 4A shows an example of a method of wiring a multiple electron source. In FIG. 4A, a total of nxc3x97m cold cathode elements are wired two-dimensionally in matrix form, with m-number of elements arrayed in the vertical direction and n-number in the horizontal direction. In FIG. 4A, numeral 3074 denotes a cold cathode element, 3072 row-direction wiring, 3073 column-direction wiring, 3075 wiring resistance of the row-direction wiring 3072 and 3076 wiring resistance of the column-direction wiring 3073. Further, Dx1, Dx2, . . . Dxm represent a feed terminals for the row-direction wiring. Further, Dy1, Dy2, . . . Dyn represent feed terminals for the column-direction wiring. This simple wiring method is referred to as a xe2x80x9cmatrix wiring methodxe2x80x9d. Since the matrix wiring method involves a simple structure, fabrication is easy.
In a case where a multiple electron beam source constructed using the matrix wiring method is applied to an image display apparatus, it is preferred that m and n each be a number of several hundred or more in order to assure display capacity. In addition, it is required that an electron beam of desired intensity be capable of being produced from each cold cathode element in order to display an image at a correct luminance.
In a case where a large number of matrix-wired cold cathode elements are driven in the prior art, the method adopted is to drive the group of elements on one row of the matrix simultaneously. Rows driven are successively changed over one by one so that all rows are scanned. In accordance with this method, drive time allocated to each element is lengthened by a factor of n in comparison with the method of scanning all elements successively one element at a time, thus making it possible to raise the luminance of the display apparatus.
One example of this is a method of driving FE-type elements disclosed by Parker et al. (U.S. Pat. No. 5,300,862). FIG. 4B is a circuit diagram for describing this method.
Numerals 2201Axcx9c2201C in FIG. 4B denote controlled constant-current sources, 2202 a switching circuit, 2203 a voltage source, 2204A a column wire, 2204B a row wire and 2205 an FE-type element.
The switching circuit 2202 selects one of the row wires 2204B and connects it to the voltage source 2203. The controlled constant-current sources 2201Axcx9c2201C supply current to each column wire 2204A. By carrying out these operations synchronously in suitable fashion, one row of FE-type elements is driven.
However, when a matrix-wired multiple electron beam source is actually driven by the above-described drive method, a problem which arises is that the intensity of the electron beam outputted from each cold cathode element deviates from the desired value. This results in unevenness or fluctuation in the luminance of the display image and, hence, a decline in picture quality.
This problem will be described in greater detail with reference to FIGS. 5Axcx9c7B. In order to avoid overly complicated drawings, FIGS. 5Axcx9c7B illustrate only one row (n pixels) of the mxc3x97n pixels. Each pixel is provided to correspond to a respective cold cathode element. The farther to the right the position is taken, the more distant the position is from the feed terminal Dx of the row wiring 3072. For the sake of simplifying the description, luminance levels are represented by numerical values, the maximum value is 255, the minimum value is 0 and the intermediate values grow successively larger by 1.
FIG. 5A illustrates an example of a desired display pattern, in which it is desired that only the right-most pixel be made to emit light at the luminance 255. FIG. 5B illustrates measurement of the-luminance of an image displayed by actually driving the cold cathode elements.
FIG. 6A illustrates another example of a desired displayed pattern, in which it is desired that the group of pixels on the left half of the row be made to emit no light (luminance 0) and that the group of pixels on the right half of the row be made to emit light at luminance 255. FIG. 6B illustrates measurement of the luminance of an image displayed by actually driving the cold cathode elements.
FIG. 7A illustrates another example of a desired displayed pattern, in which it is desired that all pixels of the row be made to emit light at luminance 255. FIG. 7B illustrates measurement of the luminance of an image displayed by actually driving the cold cathode elements.
Thus, as evident from these examples, the luminance of the actual display image deviates from the desired luminance. Moreover, if attention is directed toward the pixel indicated by arrow P in these Figures, it will be apparent that the magnitude of the deviation from the desired luminance is not necessarily constant.
As a consequence, the luminance of the displayed image is inaccurate and unstable.
Further, as shown in these Figures, undesirable light as indicated by q is emitted.
Furthermore, there are cases where pixels emit light even in rows that should not have been selected. (This phenomenon is not shown.)
For these reasons, the contrast of the image declines and picture quality deteriorates markedly.
Accordingly, an object of the present invention is to obtain a correct and fluctuation-free intensity for the electron beams produced by a multiple electron beam source having matrix-wired cold cathode elements, to prevent a deviation and fluctuation in the display luminance of an image display apparatus as well as a decline in contrast.
The foregoing object is attained by the apparatus and drive method according to the present invention described below.
Specifically, the present invention provides an electron-beam generating device comprising: a plurality of cold cathode elements arrayed in the form of rows and columns on a substrate; m-number of row wires and n-number of column wires for wiring the plurality of cold cathode elements into a matrix; and drive signal generating means for generating signals which drive the plurality of cold cathode elements one row at a time; the drive signal generating means including: current-waveform determining means for determining a current waveform, which will be passed through each of the n-number of column wires, on the basis of an externally entered electron-beam demand value; current applying means for passing the current, which has been determined by the current-waveform determining means, through each column wire; and voltage applying means for applying a voltage V1 to a row wire of a row selected from the m-number of row wires and applying a voltage V2 to all other row wires.
Further, the present invention provides a method of driving an electron-beam generating device having a plurality of cold cathode elements arrayed in the form of rows and columns on a substrate, m-number of row wires and n-number of column wires for wiring the plurality of cold cathode elements into a matrix, and drive signal generating means for generating signals which drive the plurality of cold cathode elements one row at a time; the drive method comprising: a current-waveform determining step of determining a current waveform, which will be passed through each of the n-number of column wires, on the basis of an externally entered electron-beam demand value; a current applying step of passing the current, which has been determined at the current-waveform determining step, through each column wire; and a voltage applying step of applying a voltage V1 to a row wire of a row selected from the m-number of row wires and applying a voltage V2 to the other row wires.
In order to clarify the actions of the device and drive method of the present invention as set forth above, problems encountered in the conventional drive method will be described with reference to the drawings.
As the result of exhaustive research, the inventors have discovered that when a drive pattern is altered as shown in FIGS. 5A, 6A, 7A according to the drive method of the prior art, the effective drive current which flows into a desired cold cathode element experiences a large amount of fluctuation. This will be described in connection with the conventional drive method with reference to FIGS. 8A, 8B, 9A and 9B.
FIG. 8A is a diagram showing the way in which current flows in a case where drive is performed by the method of FIG. 4B. In order to facilitate the description, a 2xc3x972 matrix is used and the wiring resistance is omitted. In FIG. 8A, CC1xcx9cCC4 represent cold cathode elements.
FIG. 8A illustrates a case in which only the element CC3 among the four elements is driven. In order to drive the element CC3, the switching circuit 2202 selects row wire Dx2 and connects it to the voltage source 2203. Meanwhile, the controlled constant-current source 2201A outputs a current IA to drive the cold cathode element CC3. The controlled constant-current source 2201B does not output any current.
In this case, the current IA is split into a current ICC3 and a current IL. Of these, the current ICC3 is a drive current which effectively acts to drive the cold cathode element CC3. The other current IL is leakage current. An equivalent circuit for calculating the current ICC3 is illustrated in FIG. 8B. To simplify the description, the resistance of each cold cathode element is indicated as Rc and the resistance of the cold cathode element CC3 particularly is encircled. When the equation shown in FIG. 8B are solved, the result obtained is ICC3=3xc2x7(IA)/4.
Next, an example in which the drive pattern is changed is shown in FIG. 9A, which shows a case in which the cold cathode elements CC3 and CC4 are driven simultaneously. The switching circuit 2202 selects row wire Dx2 and connects it to the voltage source 2203. Meanwhile, the controlled constant-current sources 2201A and 2201B output currents to drive the cold cathode elements CC3 and CC4. In a case where outputs of identical strength are sought from the cold cathode elements CC3 and CC4, it will suffice to establish the relation IA=IB. In such case no leakage current flows into the cold cathode elements CC1 and CC2. Accordingly, we have ICC3=IA, as evident from the equivalent circuit shown in FIG. 9B.
A comparison of FIGS. 8A and 9A clearly shows that regardless of the fact that the same current IA flows from the controlled, constant current source 2201A, the drive current ICC3 which effectively flows into the cold cathode element CC3 fluctuates. In other words, with the method of the prior art, the leakage current IL is not controlled and fluctuation occurs.
By contrast, in accordance with the above-described device or drive method of the present invention, it is possible to control the leakage current IL so as to have a constant magnitude. As a result, a constant drive current can be supplied to the cold cathode elements at all times even if the drive pattern is changed. The situation in the case of this invention will be described with reference to FIGS. 10A, 10B, 11A, 11B.
FIG. 10A should be compared with FIG. 8A. That is, FIG. 10A shows a case in which only the cold cathode element CC3 is driven. According to the present invention, a potential V1 is applied to a selected row wire (i.e., Dx2) and a potential V2 is applied to all unselected row wires (i.e., Dx1). In the example of FIG. 10A, a switching circuit 502 and voltage sources V1, V2 cooperate to perform this operation.
Output current IA from the a controlled constant-current source splits into a drive current ICC3 and a leakage current IL1. In the case of this invention, the leakage current IL1 is controlled by the voltages V1 and V2. A constant current IL2 flows into the cold cathode elements CC2 and CC4 as long as the output of the controlled constant-current source 501B is zero.
The drive current ICC3 and leakage current IL1 are obtained from the equivalent circuit and equations of FIG. 10B:       ICC3    =                  1        2            ⁢              xe2x80x83            ⁢              (                  IA          +                                    V2              -              V1                        Rc                          )                  IL1    =                  1        2            ⁢              xe2x80x83            ⁢              (                  IA          -                                    V2              -              V1                        Rc                          )            
FIG. 11A should be compared with FIG. 9A. That is, this is for a case in which the cold cathode elements CC3 and CC4 are driven simultaneously. In this case also the potential V1 is applied to a selected row wire (i.e., Dx2) and the potential V2 is applied to all unselected row wires (i.e., Dx1).
The drive current ICC3 and leakage current IL1 are obtained from the equivalent circuit and equations of FIG. 10B:       ICC3    =                  1        2            ⁢              xe2x80x83            ⁢              (                  IA          +                                    V2              -              V1                        Rc                          )                  IL1    =                  1        2            ⁢              xe2x80x83            ⁢              (                  IA          -                                    V2              -              V1                        Rc                          )            
Thus, according to the present invention, as evident from the foregoing examples, the leakage current IL1 can be controlled so as to be constant, as a result of which the drive current ICC3 of the cold cathode elements does not fluctuate even if the drive pattern is altered.
Accordingly, the fluctuation in output which was a problem in the prior art can be prevented. Further, since the magnitude of the leakage current can be controlled by V1 and V2, setting suitable voltage values makes it possible to prevent unnecessary electrons from being outputted by the cold cathode elements of an unselected row as a result of leakage current.
There are instances in which the leakage current flows through a parasitic conduction path besides the cold cathode elements themselves.
There are many cases in which the parasitic conduction path is formed about the periphery of the cold cathode elements or at the periphery of the member insulating the row wires from the column wires.
As a typical example of the former, consider the case of a surface-conduction electron emission element. If the surface of the substrate at the periphery of the element is soiled by electrically conductive matter 3006, a leakage current will flow (see FIG. 1).
In the case of an FE-type element, a leakage current will flow if an insulating layer 3013 is flawed or the surface of the insulating layer 3013 is soiled by electrically conductive matter 3015 (see FIG. 2).
In the case of an MIM-type element, a leakage current will flow if an insulating layer 3022 is flawed or the surface of the insulating layer 3022 is soiled by electrically conductive matter 3024 (see FIG. 3).
As a typical example of the latter, consider a case where an insulating layer provided at the solid cross section of a column wire and row wire is flawed or the surface of the insulating layer is soiled by electrically conductive matter. A leakage current will flow through the affected portion. This occurs irrespective of the type of cold cathode element.
The present invention is effective in dealing with such leakage currents ascribable to these causes.
In the electron-beam generating device according to the present invention, the current-waveform determining means comprises means for outputting the current waveform, which has been determined on the basis of the electron-beam demand value, as a voltage signal that has been amplitude-modulated or pulse-width modulated, and the current applying means comprises a voltage/current converting circuit.
In the drive method of the present invention, the current-waveform determining step comprises a step of outputting the current waveform, which has been determined on the basis of the electron-beam demand value, as a voltage signal that has been amplitude-modulated or pulse-width modulated, and the current applying step comprises a step of converting a voltage signal to a current signal.
In accordance with the device or drive method described above, once the modulated signal has been outputted in the form of a voltage signal, it is converted to a current signal. This means that the arrangement of the electrical circuitry of the controlled constant-current sources becomes very simple.
Further, in the electron-beam generating device according to the present invention, the current-waveform determining means comprises element-current determining means for determining an element current, which is to be passed through a cold cathode element of a selected row (a row to which the voltage V1 has been applied), on the basis of the externally entered electron-beam demand value and an output characteristic of the cold cathode element, and correcting means for correcting the element current determined by the electron-element current determining means.
The correcting means includes leakage-current determining means for determining a leakage-current passed through an unselected row (a row to which the voltage V2 has been applied), and adding means for adding an output value from the element-current determining means and an output value from the leakage-current determining means.
In the drive method of the present invention, the current-waveform determining step comprises an element-current determining step of determining an element current, which is to be passed through a cold cathode element of a selected row (a row to which the voltage V1 has been applied), on the basis of the externally entered electron-beam demand value and an output characteristic of the cold cathode element, and a correcting step of correcting the element current determined at the electron-element current determining step.
The correcting step includes a leakage-current determining step of determining a leakage current passed through an unselected row (a row to which the voltage V2 has been applied), and an adding step of adding an output value obtained at the element-current determining step and an output value obtained at the leakage-current determining step.
In accordance with the device or drive method described above, an accurate drive current can be supplied to a cold cathode element and, hence, an accurate output can be obtained. In particularly, the degree of accuracy can be greatly improved by correcting the leakage current, which has a great influence upon output. In particular, since leakage current can be rendered constant according to the present invention, the correction is highly effective.
Further, in the electron-beam generating device of the present invention, the leakage-current determining means includes means for applying the voltage V2 to a row wire, and current measuring means for measuring a current which flows into a column wire.
In the drive method of the present invention, the leakage-current determining step includes a current measuring step of measuring current which flows through a column wire when the voltage V2 has been applied to a row wire.
In accordance with the device and drive method described above, the precision of a correction can be raised by actually measuring the leakage current. Even if the magnitude of the leakage current varies with time, an appropriate correction can be made according to the change.
Further, in the electron-beam generating device of the present invention, the leakage-current determining means comprises a memory in which leakage values found in advance by measurement or calculation are stored.
In the drive method of the present invention, the leakage-current determining step comprises a step of reading data out of a memory in which leakage values found in advance by measurement or calculation are stored.
In accordance with the device or drive method described above, a correction can be made at high speed through a simple arrangement.
Further, in the electron-beam generating device of the present invention, the correcting means includes wiring-potential measuring means for measuring wiring potential, and means for changing amount of a correction in conformity with result of measurement by the wiring-potential measuring means.
In the drive method of the present invention, the correcting step includes a wiring-potential measuring step of measuring wiring potential, and a step of changing amount of a correction in conformity with result of measurement at the wiring-potential measuring step.
In accordance with the device or drive method described above, it is possible to apply a correction that takes into account a change in leakage current ascribable to a voltage drop caused by wiring resistance. This makes possible a further improvement in the accuracy of electron-beam output.
In the electron-beam generating device or drive method of the present invention, image information is used as the externally entered electron-beam demand information.
The above-mentioned device or drive method is ideal for use in various image forming apparatus such as an image display apparatus, printer or electron-beam exposure system.
In the electron-beam generating device of the present invention, surface-conduction electron emission elements are used as the cold cathode elements.
The above-mentioned device is simple to manufacture and even a device having a large area can be fabricated with ease.
If the electron-beam generating device of the present invention is combined with an image forming member for forming an image by irradiation with an electron beam outputted by the electron-beam generating device, an image forming apparatus having a high picture quality can be provided.
If the above-mentioned image forming apparatus has phosphors as image forming members for forming an image by irradiation with the electron beam, an image display apparatus suited to a television or computer terminal can be provided.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.